docs: def _exec_command(self, cmd):
          """Run a command on the remote SSH server.
  
          Returns:
              bytes: the output of the command, if it didn't fail
              None: if the error pipe of the command was not empty
          """
          _, out, err = self._client.exec_command(cmd, timeout=self._timeout)
          return out.read().strip() if not err.read().strip() else None